For the purposes of the present disclosure, an optical channel shall be understood to refer to a physical layer connection through an optical network between a single transmitter modem and a single receiver modem. Typically, each optical channel will be allocated a predetermined spectral band, and thus will have a known central wavelength and spectral width (which may be represented as a nominal spacing between the central wavelengths of adjacent channels). For example, ITU-T Rec. G.694.2 recommends a spectral grid of optical channels in which the respective central wavelengths are separated by 20 nm.
It is common practice to convey user data through an optical channel by modulating the user data onto a single narrow-band optical carrier light having a mean wavelength corresponding to the channel's central wavelength. However, more recent developments contemplate the use of two or more narrow-band subcarrier lights which are spaced apart within the spectral band allocated to one optical channel. Each subcarrier light may be modulated with a respective different portion of the user data. Because all of the subcarrier lights of a given optical channel lie within the optical channel's spectral band, they cannot be separately routed through the network and cannot be added or dropped independently of the other subcarriers of the channel. As such, all of the subcarriers of a given optical channel will be routed through the network together, between the transmitting and receiving modems allocated to that optical channel. This feature clearly distinguishes a multiple subcarrier optical channel from a wavelength division multiplexed (WDM) signal well known in the art.
A problem with multiple subcarrier optical channels has been identified, in that the bit error rate (BER) of the optical channel is dominated by the signal-to-noise (SNR) of the subcarrier having the lowest optical power. It is therefore desirable to equalize the respective optical power levels of the subcarrier lights within a given optical channel.